Allow viewing of next dataflow-unique identifier (#593)

Author: frankmcsherry

timely/src/dataflow/scopes/child.rs

@@ -67,6 +67,9 @@ where
     fn new_identifier(&mut self) -> usize {
         self.parent.new_identifier()
     }
+    fn peek_identifier(&self) -> usize {
+        self.parent.peek_identifier()
+    }
     fn log_register(&self) -> ::std::cell::RefMut<crate::logging_core::Registry<crate::logging::WorkerIdentifier>> {
         self.parent.log_register()
     }

timely/src/worker.rs

@@ -200,6 +200,8 @@ pub trait AsWorker : Scheduler {
 
     /// Allocates a new worker-unique identifier.
     fn new_identifier(&mut self) -> usize;
+    /// The next worker-unique identifier to be allocated.
+    fn peek_identifier(&self) -> usize;
     /// Provides access to named logging streams.
     fn log_register(&self) -> ::std::cell::RefMut<crate::logging_core::Registry<crate::logging::WorkerIdentifier>>;
     /// Provides access to the timely logging stream.
@@ -247,6 +249,7 @@ impl<A: Allocate> AsWorker for Worker<A> {
     }
 
     fn new_identifier(&mut self) -> usize { self.new_identifier() }
+    fn peek_identifier(&self) -> usize { self.peek_identifier() }
     fn log_register(&self) -> RefMut<crate::logging_core::Registry<crate::logging::WorkerIdentifier>> {
         self.log_register()
     }
@@ -526,6 +529,11 @@ impl<A: Allocate> Worker<A> {
         *self.identifiers.borrow() - 1
     }
 
+    /// The next worker-unique identifier to be allocated.
+    pub fn peek_identifier(&self) -> usize {
+        *self.identifiers.borrow()
+    }
+
     /// Access to named loggers.
     ///
     /// # Examples